1,720,997 research outputs found
Quantum Effects and Broken Symmetries in Frustrated Antiferromagnets
Full text linkWe investigate the interplay between frustration and zero-point quantum fluctuations in the ground state of two spin-half frustrated spin systems: the triangular and the J1 - J2 Heisenberg antiferromagnets. These are the simplest examples of two-dimensional spin models in which quantum effects may be strong enough to destroy the classical long-range Néel order, thus stabilizing a ground state with symmetries and correlations different from their classical counterparts. In this thesis the ground-state properties of these frustrated models are studied using finitesize spin-wave theory, exact diagonalization, and quantum Monte Carlo methods. In particular, in order to control the sign-problem instability, which affects the numerical simulation of frustrated spin systems and fermionic models, we have used the recently developed Green function Monte Carlo with Stochastic Reconfiguration. This technique, which represents the state-of-the-art among the zero-temperature quantum Monte Carlo methods, has been developed and tested in
detail in the present thesis
Development of in vitro mutagenesis and gene silencing protocols in vitis spp. to promote resistance against fungal and oomycete pathogens
Full text linkPeronospora, oidio e muffa grigia della vite sono le principali fitopatologie responsabili delle maggiori perdite economiche nel settore della viticoltura mondiale. Essendo delle malattie policicliche della porzione aerea della pianta, in condizioni ambientali favorevoli, a partire da pochi centri di infezione, i cicli secondari possono portare ad uno sviluppo epidemico esponenziale, di difficile controllo, richiedendo l’impiego di numerosi interventi preventivi con fitofarmaci. In tale contesto, la viticoltura gioca un ruolo chiave; infatti di tutti i fungicidi impiegati in agricoltura, il 65% sono imputabili alla difesa della vite, a fronte del 3% di aree agricole in Europa adibite a vigneto. Da questa consapevolezza nasce un nuovo impulso nella ricerca e nello sviluppo di strategie alternative ai mezzi di difesa chimici, impiegando ad esempio approcci genetici volti ad incrementare la resistenza nell’ospite. In codesto scenario, uno dei principali obiettivi della tesi è stato quello di ampliare il panorama di variabilità genetica, attraverso l’applicazione di tecniche già conosciute e impiegate in programmi di miglioramento genetico, come la mutagenesi chimica e la variabilità somaclonale, o l’associazione del meccanismo di RNAi al contesto viticolo, favorendo una maggiore tolleranza nell’ospite, mantenendo inalterata l’identità varietale. Inoltre, l’importanza dei processi di morfogenesi in vitro, tra i quali organogenesi ed embriogenesi somatica, è stata enfatizzata come strumento essenziale per l’applicazione degli approcci biotecnologici su differenti cultivar e portinnesti di vite. Molteplici linee di ricerca hanno dimostrato come il successo della rigenerazione in vitro delle piante e la competenza alla trasformazione genetica, dipendano ampiamente dal genotipo selezionato della specie di interesse. Pertanto, la rigenerazione in vitro associata alla trasformazione mediata da Agrobatterio, sono state efficientemente combinate per l’induzione del silenziamento genico post-trascrizionale, mediato da molecole di RNA a doppio filamento (dsRNA), attraverso l’introduzione di un costrutto genico RNAi in vite (Host Induced gene silencing – HIGS), mirato al silenziamento di geni target di Botrytis cinerea e Plasmopara viticola che codificano per effettori di virulenza. Parallelamente, il sistema complesso dell’interazione ospite-patogeno può essere alterato in favore dell’ospite, attraverso l’ottimizzazione di un nuovo approccio RNAi che si basa sull’applicazione esogena delle medesime molecole di RNA a doppio filamento sulla superficie delle piante, per limitare la virulenza dei patogeni fungini e oomiceti. L’efficacia di tale sistema esogeno è stata testata impiegando come target due geni di Botrytis cinerea, ed è risultata efficace per ostacolare e prevenire la muffa grigia, ritardando la sua aggressività su acini e foglie di vite.Downy mildew, powdery mildew, and grey mold are the phytopathological diseases primarily responsible for economic losses in grapevine culture worldwide. These are polycyclic diseases of the aerial plant system. Starting from a few primary infection centres, under favourable environmental conditions, secondary cycles can lead to exponential epidemic developments which are difficult to contain, forcing the use of numerous preventive treatments, like phytochemicals. In this context, viticulture plays a fundamental role; in fact, the 65% of all fungicides used in agriculture are applied to vineyards, compared to the 3% of agricultural area occupied in Europe. This awareness has led to a new impulse towards the research and the development of alternative strategies to chemical means of protection, like the exploitation of genetic approaches, aimed at increasing the resistance of the host. In this scenario, one of the main purposes of this thesis has been to broaden the panorama of genetic variability, through techniques already known and used in the genetic improvement programs, such as chemical mutagenesis and somaclonal variability, or through the application of RNA interference (RNAi) approaches applied to the viticultural context, by introducing new host resistances, while maintaining the varietal identity. Furthermore, the importance of in vitro morphogenesis, in terms of de novo shoot organogenesis and somatic embryogenesis, has been emphasized as essential tool for the application of biotechnological approaches to be used on different grapevine cultivars and rootstock. Several lines of research have shown that the success of in vitro plant regeneration and the competence of genetic transformation greatly depends on the genotype of the species of interest. Therefore, both in vitro regeneration and Agrobacterium-mediated transformation systems have been efficiently endorsed for the induction of a post-transcriptional gene silencing mechanism mediated by double-stranded RNA (dsRNA) molecules, by inserting RNAi-based gene constructs in grapevine (Host Induced gene silencing – HIGS), which target genes of Botrytis cinerea and Plasmopara viticola that act as virulence effectors. Parallelly, the complex system of the pathogen-host interaction can be altered in favour of the latter through the optimization of a new RNAi approach, which is based on the exogenous application on the plant surface of the same dsRNA molecules to reduce the virulence of fungal and oomycetes pathogens. The efficacy of the exogenous application of dsRNA molecules targeting genes of Botrytis cinerea has been tested and considered suitable to obstruct and prevent the disease and delay its appearance on grapevine leaves and berries
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Adventitious Shoot Regeneration from In Vitro Leaf Explants of the Peach Rootstock Hansen 536
Full text linkIn the present study, an efficient system for the in vitro regeneration of adventitious shoots from the peach rootstock Hansen 536 leaves has been established. Twenty regeneration media containing McCown Woody Plant Medium (WPM) as a basal salt supplemented with different concentrations and combinations of plant growth regulators (PGRs) were tested. Expanded leaves along with their petiole from 3-week-old elongated in vitro shoot cultures were used as starting explants. The highest regeneration rate (up to 53%) was obtained on WPM basal medium enriched with 15.5 μM N6-benzylaminopurine (BAP). The influences on leaf regeneration of the ethylene inhibitor silver thiosulphate (STS) and of different combinations of antibiotics added to the optimized regeneration medium were also investigated. The use of 10 μM STS or carbenicillin (238 μM) combined with cefotaxime (210 μM) significantly increased the average number of regenerating shoots per leaf compared to the control. In vitro shoots were finally elongated, rooted and successfully acclimatized in the greenhouse. The results achieved in this study advances the knowledge on factors affecting leaf organogenesis in Prunus spp., and the regeneration protocol described looks promising for the optimization of new genetic transformation procedures in Hansen 536 and other peach rootstocks and cultivars
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Correction to: RNA interference-based strategies to control Botrytis cinerea infection in cultivated strawberry
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